The present invention relates to a three color electrophotographic or electrostatic copying process.
Color electrostatic copying machines which produce full color copies are known in the art. These are generally of two types. The first type comprises a single photoconductive drum or belt which is exposed to a light image of an original document three times through filters of three primary colors respectively. After each imaging operation, a toner substance of a corresponding color is applied to the drum to form a color toner image which is transferred to a copy sheet. In this manner, three color toner images are sequentially formed on the drum and transferred to the copy sheet in register to produce a color copy. Often, a fourth black toner image is formed and transferred to the copy sheet in register with the three color toner images.
In such a copy machine it is essential that the toner images be transferred to the copy sheet in perfect register. The control mechanism for such a copying machine is therefore intricate and expensive. The three or four imaging operations for each copy require a disproportionate amount of time, making the process very slow.
The second type of color copying machine is much faster in operation but also much more expensive to manufacture. Such a copying machine comprises three or four photoconductive drums or belts. The original document is passed over all of the drums in one scanning movement, sequentially imaging the drums through three respective primary color filters. A toner development unit is associated with each drum. The copy sheet is fed through the machine in one pass, with the toner images being transferred thereto in register through sequential engagement with the drums.
In addition to the increased cost of the three or four drums compared to only one drum or belt in the first type of color copying machine, an intricate mechanism is also required in the second type of machine to ensure perfect register of the three of four toner images on the copy sheet.
A full color copying machine is unnecessary in many business operations where only commercial documents are copied, since such documents generally only comprise the colors black and red, in addition to a white background. This is because accounting records and the like generally contain credit entries in black and debit entries in red. Since in many such documents the debit and credit entries may be distinguished from each other only by the color of ink many offices have purchased or leased full color copying machines for copying such records. The full color copying capability is wasted since it is only necessary to distinguish red from black on the copies.
Electrostatic copying processes which are capable of producing two color copies in a unique and simplified manner are disclosed in Japanese patent application numbers 52-68343, 52-82660, 53-13273, 53-18754, 53-50664, 53-55789 and 53-60211 which are assigned to the same assignee as this application.
Such a process comprises the steps of providing a material including a first photoconductive layer which is insensitive to light of a predetermined color, a second photoconductive layer which is sensitive to light of the predetermined color and a transparent insulating layer, forming an electrostatic charge pattern in the material in such a manner that electrostatic charges of opposite polarities are formed at outer surfaces of the first and second photoconductive layers respectively, radiating a color light image onto an outer surface of the material and applying two toners of different colors which are charged to opposite electrostatic polarities respectively to the outer surface of the material.
The present invention constitutes a major improvement over the prior process in that it enables copying in three colors such as black, red and green or black, red and blue.